I have to use interrupt capture for timer 1 (0 is used by arduino core functions). The problem is that D5 is already used
as the slave select pin for flash memory. I think it could be a nice idea to remap the slave select on D4 . The alternate function s
of D4 are related to timer 0, so it will obviously be used as a standard digital out/in.
And then the timer 1 input capture , for frequency measurement for example, would be available.

I believe input capture (ICP) is pin 8 on the Arduino Uno pinout. If you're measuring a fairly slow signal, under a few kHz (or 10000 RPM motor), pin 8 with a library like FreqMeasure probably makes more sense. For full disclosure, I'm the author of FreqMeasure....

But for higher frequency, you probably do need pin 5, the external clock input for timer1, and perhaps my FreqCount library?